The tension in an endless drive cable of the type used to drive an aerial tramway (e.g., chairlift, gondola, tram), ski lift or the like must be kept within certain ranges for efficiency and safety of operation. Drive cables are subjected to a wide range of conditions which affect the tension forces in the cable. The passenger load on a drive cable can vary from essentially the weight of the cable and the chairs, gondolas, etc. to a fully loaded condition Moreover, these loads and the tension forces in the cable are periodically varying as passengers and/or cargo load and unload from the aerial tramway. Still further, aerial tramways may be called upon to operate under widely varying temperature conditions, e.g., from 60.degree. to 70.degree. F. during summer time operation, or even higher, down to -10.degree. to -20.degree. F., or even lower, during winter operation.
Perhaps the most often employed method of adjusting the tension in the endless drive cable in an aerial tramway or the like is to mount one of the bull wheels of the tramway on a carriage that is movable in a direction enabling elongation or relaxation of the cable. Most usually the carriage for the bull wheel is attached to a large counterweight which applies a constant force to the carriage, and accordingly the cable. U.S. Pat. No. 3,951,073 is a good example of a counterweight cable tensioning system for a chairlift.
There are, however, certain drawbacks to use of a counterweight tensioning apparatus. First, the counterweight often has to be undesirably massive and unwieldy, for example, as large as a sixteen ton block of concrete. Second, it is sometimes necessary or desirable to be able to raise or lower the vertical height of the horizontal bull wheel. Such raising and lowering can dramatically change the relationship with respect to the counterweight and the bull wheel or require raising and lowering of the counterweight and its support structure, a difficult task. Third, while the counterweight system theoretically induces a constant tension force in the cable, substantial problems have been encountered with the counterweight pulleys, guides and the like becoming frozen or their movement otherwise impeded under the adverse environmental conditions, and the system inherently has high inertia and slow reaction time to dynamic loads.
Another approach has been to drive the movable carriage to which the horizontal bull wheel is mounted by a hydraulic cylinder so as to enable variation of the tension forces in the drive cable by hydraulics. U.S. Pat. No. 3,377,959 is typical of this type of approach. While hydraulic systems have the advantage of avoiding the mass of large counterweight systems, they also have substantial disadvantages. Hydraulic systems are basically closed systems in which the hydraulic fluid must be periodically pumped into the cylinder, with the excess fluid returning to a reservoir for later use. Thus, the system must have a periodically operating pump, and under many aerial tramway regulating codes, there must be an auxiliary pump to back up the first pump in the case of failure. Such hydraulic systems can be operated manually or by automatic controls. Manual systems lead to the undesirable buildup of tension forces in the cable, while automatic hydraulic controls are inherently relatively complex and less sensitive than is otherwise desirable. In hydraulic systems, the imcompressibility of the hydraulic fluid can result in reaction times in the system which are far slower than the dynamic loading conventionally experienced by the cable. U.S. Pat. No. 3,987,735 is an example of a hydraulic cable tension controlling system for railroad car spotting. While adequate for its purposes, this system is not suitable for nor designed to be employed in a tramway cable tensioning maintenance and control system.
Some attempts have been made to enhance hydraulic systems by adding a gas-over-fluid approach, such as is shown in U.S. Pat. No. 888,439 or even an air bladder in combination with the hydraulic fluid to attempt to cushion shock loading. These systems are still faced with the problems of complex automatic controls, multiple pumps and leakage of the hydraulic fluid from the system, because the gas is used as a shock absorber, but not the basic working medium.
Pneumatic apparatus have been previously employed as part of a variety of non-aerial tramway systems which employ cable under tension. These systems often do not experience the same wide range of loading conditions as does an aerial tramway system,and the pneumatic apparatus have accordingly been designed to accomplish different functions. In U.S. Pat. No. 1,281,323, for example, a steam actuated cylinder provides a tension force on a cable, which when overcome, allows displacement of a pulley system to indicate that a tower is in danger of tipping over. In U.S. Pat. No. 3,661,090 a pneumatic cylinder is employed in connection with a cable system to act essentially as a shock absorber. Tension adjustment is achieved through a complex winching system.